RPM gauge warning light

I just got a 94 Dakota 4X4, 5sp, V8 and it didn't come with a user's manual...I am getting a warning light (I think) in the RPM gauge at random times. It is an orangish/yellowish triangle (with the point pointing up to the RPMs) and it comes on without any pattern I can discern. At first I thought it meant "you can shift now" as it would pop up when RPMs got a little high, esp in low gears, but I tried purposefully letting them get high sometimes and the light wouldn't come on. Or conversely, letting them drop low and it would come on sometimes and not others... Is this a true "warning light" (ie, something in the engine is wrong) or is it a "help" light (ie it is telling me to shift or something else) and what is it telling me or warning me about?? Also, was running low on oil, and wondered if it needed a certain weight oil or just 10/30 ?? Another side note, it came with a Pioneer XM satelite radio and no manual to go with the radio..anyone here have anything like that? I can't get ANY local radio stations (the CD player works great though and the truck has an AWESOME sound system...) and I don't know if I "have to" sign up for XM or if there is a way to get it back onto the antenna?? I also can't get the clock set...sigh... Great truck though otherwise :) thanks for any help! Nat